By Dr. Dineshani Hettiarachchi Sirisena
Dengue is on the rise again. According to latest trends published by the Epidemiology Unit of the Ministry of Health in Sri Lanka, the total number of reported dengue cases from January to July 1 this year is 24,472.
Among them, 4,790 cases are reported solely from the Colombo area. It is no secret that climatic factors play a pivotal role in dengue incidence with the highest reported cases usually coinciding with rainfall. To curb the current situation, the Colombo Municipal Council (CMC) is looking at new vector control methods.
We spoke to Cinnamon Gardens, Borella, Medical Officer of Health Dr. Subash Mendis, who is in charge of an area under the preview of the CMC. He is also in charge of the latest programmes being implemented.
The pilot project was carried out by the CMC with 1,000 ovitraps donated by Rotary International. Dr. Mendis stated within the first four months of its installation, they had already experienced a dramatic fall in the number of dengue cases in the study area. Hopefully, this can be implemented islandwide without delay.
What is vector control?
It is preventing or reducing virus transmission by controlling the mosquito or interrupting human-mosquito contact.
The World Health Organisation (WHO) promotes a strategic approach known as Integrated Vector Management (IVM) to control mosquito vectors, including those of dengue.
The dengue virus is transmitted by female mosquitoes mainly of the species Aedes aegypti and, to a lesser extent, Aedes albopictus. This mosquito also transmits chikungunya, yellow fever, and Zika infections.
Unlike many other mosquito species, A. aegypti is a day-biting mosquito, and often feeds on multiple hosts during a single gonotrophic cycle.
Females preferentially lay eggs in manmade or artificial containers including water tanks, flower vases, pot plant bases, discarded tyres, buckets, or other containers typically found around or inside the home.
Eggs are laid on or near the water surface in containers and, once embryonated, can withstand desiccation for up to one year. One female mosquito lays around 200-300 eggs in several breeding sites (around 10-12) in one breeding cycle, and has about five such cycles in a lifespan. This information is particularly important in community vector control programmes, as finding a single breeding site can indicate several more in the vicinity.
Vector control methods presently used in Sri Lanka
The Sri Lankan Government uses integrated vector control strategies including environmental management, biological control, chemical control, and use of personal protection methods in order to reduce the dengue burden. According to Dr. Mendis, several methods are being employed and they can be broadly divided in to larvicidal agents and adulticidal agents where in the former, the larval form of the mosquito is targeted and in the latter, the adult mosquito is targeted.
· Environmental management – minimising vector breeding and/or vector destruction through methods including house-to-house inspection and removing mosquito breeding sites.
· Ensuring proper waste disposal – avoiding mosquito-human contact by vector proofing windows in tropical houses with mosquito nets and/or screens on doors and windows.
Biological vector control uses predators, parasites, or pathogens to reduce mosquito larvae such as freshwater fish and crustaceans.
Bacterial larvicides, Bacillus thuringiensis subsp. israelensis (Bti), is a bacterium developed using recombinant DNA technology and introduced to areas where there is water stagnation such as at construction sites.
A potent larvicide based on the active ingredient temephos (Abate®) effectively manages a broad spectrum of mosquito larvae before they hatch.
In Sri Lanka, space spraying (fogging) is widely used for dengue vector control.
However, space spraying is recommended for vector control only in emergency situations to suppress ongoing epidemics or to prevent an incipient one.
Larvicide-treated ovitraps – DENGUETRAP
Mosquito traps are regarded a cost-effective and environment-friendly strategy in controlling dengue compared to other chemical and biological methods. But in Sri Lanka, traps are seldom used as both health authorities and laypeople are not familiar with mosquito traps.
There are mainly 2 types of mosquito traps; adult mosquito traps and ovitraps.
As the name suggests, adult mosquito traps attract adult mosquitoes and destroy them.
Ovitraps attract female mosquitoes towards the trap to lay eggs and the emerging larvae get killed by a chemical within the trap.
DENGUETRAP is an ovitrap invented by Public Health Inspector Sirira Basnayake six years ago, and it was further modified by Dr. Mendis. It is specially designed to attract Aedes mosquitoes and kill its larvae.
Advantages of DENGUETRAP
- Cost effectiveness – you need only one DENGUETRAP per average-size house (10 perches) which costs approximately Rs. 1,500
- Does not need electricity or any other power source
- Can be used both indoors and outdoors
- Does not emit harmful chemicals to the environment
- It needs only minimal maintenance after proper installation for a six-month period. Once in six months it requires some cleaning and re-applying of the residual chemical
- One instrument can be used for several years
DENGUETRAP does not contain any pesticides. Hence, it can be considered an environmentally friendly biological control method.
“However, during a pesticide evaluation meeting held at the National Dengue Control Unit, several entomology and parasitology experts suggested using pyriproxyfen, an insect growth inhibitor used to control Aedes mosquitoes, as an additional control measure inside the trap.
Since we have already done some experiments using pyriproxyfen with the DENGUETRAP and are convinced about its efficacy, we decided to add pyriproxyfen to the trap during the ongoing pilot project in central Colombo, New Bazaar area,” stated Dr. Mendis.
DENGUETRAP is ideal for construction sites, schools, government and private institutes with a large land area, and also for households.
It takes only three to four weeks to reduce the mosquito population of a certain geographic area and the effects are sustainable over a long period with minimum cost.
DENGUETRAP – mechanism of action
It lures female adult mosquitoes into the trap to lay eggs. Aedes mosquitoes get attracted to small, dark water containers to lay eggs. The mosquitoes are attracted to water containers where there is degrading organic matter in which a large number of bacteria is present. Aedes mosquitoes have sense organs which will detect certain types of fatty acids associated with these bacteria, from a distance.
This trap contains a special mixture of degrading organic matter consisting of hay, animal dung, and algae in a separate compartment. This special mixture will emit the odour of degrading organic matter into the environment, which will attract female Aedes mosquitoes searching for water containers to lay eggs. This trap provides an ideal environment for the Aedes mosquitoes to breed.
In the presence of water and favourable conditions, mosquito eggs will hatch within 24 hours. The emerging larvae will be moved to the bottom of the water compartment by their specific up and down vertical movements with the help of the angled surface.
There is a thin larvicidal oil layer on top of the water compartment which is at the bottom. Once the larvae reach this compartment, they will try to reach the surface to get oxygen.
The oil layer will effectively block the larvae from obtaining oxygen from the surface. Furthermore, the larvicidal oil itself is toxic for the larvae. Hence, all the larvae will get killed within 15 minutes.
During the process of laying eggs, Aedes mosquitoes release the mosquito oviposition pheromone, which acts as a chemical signal for the other female mosquitoes to lay eggs in the same container. Water used in the DENGUETRAP will not be changed once the device is installed. Hence, mosquito oviposition pheromone levels in the water will progressively increase, luring more gravid mosquitoes into the trap.
A layer of pyriproxyfen is applied on the inner surface of the trap where the female Aedes mosquitoes land to lay eggs. Pyriproxifen is an insect growth regulator which prevents Aedes larvae from becoming adult mosquitoes, and these will get killed in the larval stage. The addition of Pyriproxifen will ensure adult mosquitoes won’t emerge from the trap, even if it is not maintained properly.
Another very important action of pyriproxyfen is tiny particles of this chemical attach to the legs of the female mosquitoes when it lays eggs in the trap. These tiny particles will then get transferred to other water containers in the environment via the female Aedes mosquitoes as they usually lay their eggs in 10-12 different places. As such, by using pyriproxyfen in this trap, it enables the transfer of the chemical to the other breeding places in the environment, further preventing emergence of mosquitoes from those places too.
How is it more effective than other traps?
Most of the commercially available traps focus on killing adult mosquitoes. In areas where there is high mosquito density, adult mosquito traps are highly ineffective, only giving psychological relief to the user. As long as the mosquito breeding places are functioning undisturbed, mosquitoes will keep on breeding, feeding more and more adult mosquitoes to the traps.
Most other ovitraps do not use potent attractants, which only attract gravid mosquitoes. DENGUETRAP contains a special natural formula made out of algae and hay to lure gravid mosquitoes into the trap to lay eggs. Furthermore, most of the ovitraps need regular maintenance where chemicals need to be applied monthly to kill larvae. The major drawback is if the user fails to insert the chemical regularly, the trap can turn in to a mosquito breeding ground.